A coordinate measuring machine (CMM) is a computer-controlled device for measuring the geometrical dimensions of an object. Object geometric features are measured using a probe moved by the machine in X, Y, and Z spatial coordinate axes. The probe, also referred to as a stylus, may be brought into contact with the object to obtain a measurement of physical coordinates at a point of contact. Bridge-type CMMs commonly have a granite table defining an X-Y plane, a gantry mechanism supported by a pair of vertical legs to extend transversely over the table, and a probe head extending downward from the gantry. The probe head is movable transversely along the gantry in opposite X-axis directions. The gantry with the probe head is movable forward and backward along the length of the table in opposite Y-axis directions. The probe head is also moveable vertically up and down relative to the gantry and table surface in opposite Z-axis directions. In some CMM designs, the probe head may be provided with one or more rotational degrees of freedom in addition to the X-Y-Z motion capability.
The stylus may be mounted onto the probe head by an adapter plate. The stylus may be coupled to the adapter plate through an assembly of one or more rigid extension arms configured so that the stylus can reach features of interest on the object that is being measured. The adapter plate may be mounted onto the probe head of the CMM in a manner allowing the CMM to swap among different adapter plates having different stylus arm configurations. For example, as will be understood by persons familiar with CMMs, the adapter plate may be mounted onto the probe head of the CMM by means of an electromagnet, and a rack holding several adapter plates each with a different stylus configuration is accessible to the probe, whereby an adapter plate on the probe head may be deposited in an open space in the rack by de-energizing the electromagnet, and a different adapter plate may be mounted onto the probe head by positioning the probe head over the desired adapter plate and energizing the electromagnet.
A common arrangement is depicted in FIGS. 1A and 1B, wherein a CMM probe head 2 has an adapter plate 3 mounted thereon such that a central axis of the adapter plate coincides with a vertical Z-axis along which probe head 2 travels. A rigid plate extension 4 is coupled to adapter plate 3 by a first coupling cube 5 to extend downward in the Z-axis direction toward a table surface 1 of the CMM. A rigid stylus extension 6 is coupled to a distal end of plate extension 4 by a second coupling cube 7 such that the stylus extension extends in an X-Y plane of the CMM parallel to table surface 1. A contact stylus 8 is carried at a distal end of stylus extension 6. The first coupling cube 5 is releasably clamped on adapter plate 3 so that its angular orientation relative to the adapter plate about the central axis of the adapter plate is adjustable. In a known arrangement, several adjustment screws on a rear side of the adapter plate may be loosened to permit angular adjustment and then tightened to secure the first coupling cube 5 in a chosen angular orientation. As will be apparent, this permits angular adjustment of the entire stylus assembly connected to coupling cube 5, including plate extension 4, second coupling cube 7, stylus extension 6, and contact stylus 8. Thus, the stylus extension 6 and contact stylus 8 may be orientated in any direction in the X-Y plane depending upon the angular orientation of the first coupling cube 5 relative to the adapter plate.
Before taking measurements with the CMM, it may be necessary to calibrate the CMM by angularly aligning stylus extension 6 relative to adapter plate 3 such that the stylus extension 6 extends along one of the horizontal axes of the CMM, for example the Y-axis along which the gantry travels. Further reference is made now to FIGS. 1A and 1B to describe a known prior art method of angularly aligning a stylus extension relative to an adapter plate. As mentioned above, stylus extension 6 is angularly adjustable about a central axis of adapter plate 3 coinciding with the vertical Z-axis of probe head 2. To perform such an adjustment, adapter plate 3 is removed from probe head 2, and the adjustment screws at the rear of adapter plate 3 are loosened to permit rotation of first coupling cube 5 and the depending structure relative to adapter plate 3. With the screws loosened, the stylus assembly—i.e. first coupling cube 5, plate extension 4, second coupling cube 7, stylus extension 6, and stylus 8—is rotated as a unit about the central axis of adapter plate 3 to a new angular position. Once a desired angular position is reached, the adjustment screws are tightened to fixedly secure the rotated stylus assembly relative to adapter plate 3.
In the example illustrated in FIGS. 1A and 1B, the goal is to align stylus extension 6 to extend solely along a Y-axis direction. To determine the extent to which stylus extension 6 varies from the desired alignment direction, a vice 9 is situated at a set position on table surface 1 and a gauge pin 10 is clamped in vice 9 to provide a fixed (stationary) reference location. The CMM is operated to move probe head 2 to bring a distal end portion 6A of stylus extension 6 into contact with gauge pin 10 as shown in FIG. 1A, whereby the CMM can obtain a first spatial data point comprising X, Y, and Z axis coordinates. The CMM is also operated to move probe head 2 to bring a proximal end portion 6B of stylus extension 6 into contact with gauge pin 10 as shown in FIG. 1B, whereby the CMM can obtain a second spatial data point comprising X, Y, and Z axis coordinates. By comparing the X and Y coordinates of the first spatial data point to the X and Y coordinates of the second spatial data point, an indication of alignment may be ascertained. For example, if the difference between the two measured X coordinates is less than or equal to 0.001 inches per inch of extension along the Y-axis, then alignment of stylus extension 6 may be deemed acceptable. If not, an angular adjustment procedure must be carried out as described in the preceding paragraph, and then the CMM must be operated to check the alignment again as described in this paragraph. This procedure must be repeated until an acceptable alignment is achieved. It is not uncommon to repeat these steps as many as five times or more to achieve an acceptable alignment calibration.
As is understood by CMM operators, this process is extremely time consuming, sometimes taking hours to complete, and depends upon the skill and experience of the operator. A more efficient solution is needed.